This invention pertains to sensing the direction of the gravity vector in the vicinity of the inclinometer. More particularly, the invention pertains to measurement of inclination with respect to vertical, particularly as applied for directional determination during well drilling
The prior art, particularly in connection with directional determination during well drilling, has involved the use of accelerometers. These accelerometers as previously used for borehole inclination measurement are fragile, expensive, temperature sensitive and large. In accordance with the present invention, a very rugged, low-cost, temperature-insensitive, low-power, non-magnetic miniature inclinometer is provided for use in such applications and otherwise.
The basic physical principle employed in an inclinometer to sense the position of the gravity vector is that a mass not otherwise constrained will always come to rest in a condition of minimum potential energy. In the case of a mass constrained to remain within a fixed volume, it will move to occupy the lowest point in the volume. This is illustrated by the plumb bob, which is constrained to remain within a sphere whose radius is defined by the length of its supporting line, but will always come to rest with the bob directly below the point of suspension: the lowest point in the sphere of possible points. The direction of the gravity vector in the vicinity of the mass is defined as the normal to the constraining surface at the point where the mass contacts the surface. Again, for the plumb bob example, the constraining surface is a sphere, and the gravity direction is the normal to the sphere at its lowest point. Since all normals to a sphere are also radii of the sphere and therefore pass through its center, the sensed direction of the gravity vector is coincidental with the plumb bob support line--a well known practical fact.
Many examples exist in the prior art of using a constrained mass for purposes of sensing gravity, notable among which are the curved vial and ball of an aircraft bank indicator, and the floating bubble of a carpenter's level. For both the aircraft bank indicator and the carpenter's level, the readout is optical; that is, one visually determines the inclination based on the position of the ball or bubble with respect to index markings. Also, neither of these prior art devices is adapted to provide proper indications of inclination over a full range of 36o.degree. movement.
The prior art has recognized the desirability of providing an inclinometer arrangement in which an electrical, as opposed to visual, readout is provided. One such general type of prior art arrangement involves parallel resistive tracks on which a spherical ball rolls. Such arrangements demand at least two points of contact between the parallel tracks and ball and complete a circuit of some sort through the ball whereby the ball position is indicated by means of a voltage drop or the like. These devices tend to be relatively insensitive in that friction is substantial due to the necessity of two points of contact between the ball and track. Furthermore, unless the ball is constrained in some manner, the device will not operate over 360.degree. because the ball will fall off when the tracks are inverted. If the ball is constrained between two diametrically opposed tracks, then presumably it could be tilted over. However, what would result is a relatively insensitive device because of the greatly increased friction to which the ball would be subjected.
Another type of prior art arrangement utilizes a capacitive coupling. Typically, there is an assembly including stationary plates and a movable element on a cantilevered arm. As the assembly is rotated, the force of gravity tends to rotate the movable element so as to change the effective capacitance. The capacitance is measured in a balanced bridge or the like and is used for calculating the magnitude or direction of the force of gravity.
None of these prior art arrangements as discussed above function well in hostile environments, such as temperature extremes and shock and vibration.